Electrostatic printing upon an image recording medium comprises the formation of a latent, electrostatic image by the selective creation of air ions and the deposition of those ions of a given sign (usually negative) at selected pixel locations on the recording medium. The aggregate of the charged pixel areas forms an electrostatic latent (i.e. non-visible) image which is subsequently made visible at a development station. Development may be accomplished by passing of the recording medium, bearing the latent image, into contact with a liquid solution containing positively charged dye particles in colloidal suspension. The dye particles will be attracted to the negatively charged imaging ions so as to render the image visible. The visual density of the image thus developed will be a function of the potential or charge density of the electrostatic image.
Two types of image recording media in common usage are paper and film. The paper is specially treated so that its bulk will be electrically conductive and is overcoated with a thin dielectric coating on its image bearing side. The film comprises a dielectric substrate (such as Mylar.RTM.) overcoated with a very thin, semi-transparent intermediate conductive layer and a surface dielectric layer upon its image bearing side. To write on the media, electrical contact must be made to bleed off electrical charge. For film, electrical contact is made by conductive stripes painted near the edges of the media which penetrate the dielectric layer to make electrical contact with the conductive inner layer of the media. When writing on paper media, electrical contact is made directly to the backside of the paper. The backplate portion of the writing potential is established in the paper conductive layer by direct contact thereof with the conductive counter electrodes, that is, by essentially resistive coupling. When writing on film, the backplate portion of the writing potential is established in the intermediate conductive layer by capacitive coupling, through the Mylar substrate, between the intermediate conductive layer and the counter electrodes.
Conventionally, an electrostatic image may be formed upon the thin surface dielectric layer of a paper recording medium by passing the recording medium between a recording head, including an array of recording stylus electrodes, and a counter electrode comprising an array of complementary counter electrode segments. A charge is applied to selected pixel locations on the recording medium by the coincidence of voltage pulses applied to opposite surfaces thereof, by the stylus electrodes and the counter electrodes. When the potential difference between the stylus electrodes and the conductive layer of the recording medium is large enough to cause the voltage in the air gap between the stylus electrodes and the surface of the dielectric layer to exceed the breakdown threshold of the air, the air gap becomes ionized and air ions, of the opposite sign to the potential of the conductive layer, are attracted to the surface of the dielectric layer. As the dielectric surface charges up, the voltage across the gap will decrease to a value below the maintenance voltage of the discharge. At that time, the discharge extinguishes, leaving the dielectric surface charged. A potential difference of about 600 volts (about 800 volts for film) is required to establish a discharge. Of that threshold potential, about -200 volts is imposed on the stylus electrodes contemporaneous with the application of about +400 volts (+600 volts for film) on the counter electrodes.
Electrostatic recorders may be typically from 11 inches to 44 inches wide, and in some cases even as wide as 72 inches. Therefore, the writing head stylus array which extends fully across this width may have as many as 2000 to over 17,000 styli (at resolutions of 200 to 400 dots per inch). Because of this very large number of styli it is ordinarily not economically attractive to use a single driver per stylus, and a multiplexing arrangement is commonly used in conjunction with the above-described electrostatic discharge method. The styli in the writing head array are divided into stylus electrode groups (each group being about 0.64 inch to 2.56 inches long) so that each may consist of several hundred styli. The stylus electrodes are wired in parallel with like numbered styli in each group being connected to a single driver and carrying the same information. Writing will only occur in the stylus group whose complementary counter electrode is pulsed.
In U.S. Pat. No. 4,424,522 (Lloyd et al) entitled "Capacitive Electrostatic Stylus Writing With Counter Electrodes" there is disclosed a backplate electrode assembly which is conformable to the arcuate crown of the recording head. A structure of this type is illustrated in FIGS. 1 and 2, and is more fully described below. It comprises a plurality of segments of an electrically resistive material mounted upon an elongated, U-shaped, support bar so as to be electrically independent. The segments are anchored to the support bar and are stretched over the channel thereof within which is provided a resilient member for urging the surface of the resistive material into intimate contact with the recording medium. In its commercial application, in electrostatic printer/plotters manufactured by the assignee of the present patent application, the resilient member comprises a strip of foam and an oil-filled bladder for urging the segmented backplate electrodes toward the writing head.
The complexity of the biasing elements of the backplate electrode structure described above increases the cost of manufacture. Furthermore, uniform wrapping tension of each segment upon the support bar is difficult to achieve, and insufficient tension can result in curling of the segment edges which allows debris and chaff to collect in the gaps and thus provide a shorting path. Non-uniform tension along the writing line can also cause image intensity variations across the plot and wear variations across the writing head which result in image striations, i.e. visible striping on the printed image extending in the direction of movement of the recording medium. As the pressure applied by the biasing elements against the recording head increases, so does the likelihood of flaring because flare writing increases with pressure as the media's surface abrades the ends of the styli. Flaring is a phenomenon caused by non-uniform electrical discharge which results in non-uniform electrostatic image spots being created on the recording medium.
Therefore, the objects of the present invention are to overcome these shortcomings by providing a counter electrode in which the biasing element, for urging the electrically conductive material against the recording head, is of simple and inexpensive construction and will conform to the shape of the recording head. Furthermore, it would be desirable if the counter electrode could provide a non-uniform contact pressure sufficient to conform the media to the recording head surface with a minimum force being applied along the nib line.